permit the establishment of a field with the name of size or other

Example:
------------

### model
```
class Example(Document):
    size = ReferenceField(Size, verbose_name='Size')
```
### query
```
examples = Example.objects(size=instance_size)
```
### caused an error

```
File ".../mongoengine/queryset/transform.py", line 50, in query
if parts[-1] == 'not':
IndexError: list index out of range
```